4,897 research outputs found
Structural load challenges during space shuttle development
The challenges that resulted from the unique configuration of the space shuttle and capabilities developed to meet these challenges are described. The methods and the organization that were developed to perform dynamic loads analyses on the space shuttle configuration and to assess dynamic data developed after design are discussed. Examples are presented from the dynamic loads analysis of the lift-off and maximum dynamic pressure portion of ascent. Also shown are orbital flight test results, for which selected predicted responses are compared to measured data for the lift-off and high-dynamic-pressure times of ascent. These results have generally verified the design analysis. However, subscale testing was found to be deficient in predicting full-scale results in two areas: the ignition overpressure at lift-off and the aerodynamics/plume interactions at high-q boost. In these areas, the results of the flight test program were accommodated with no impact to the vehicle design
Numerical models for the circumstellar medium around Betelgeuse
The nearby red supergiant (RSG) Betelgeuse has a complex circumstellar medium
out to at least 0.5 parsecs from its surface, shaped by its mass-loss history
within the past 0.1 Myr, its environment, and its motion through the
interstellar medium (ISM). In principle its mass-loss history can be
constrained by comparing hydrodynamic models with observations. Observations
and numerical simulations indicate that Betelgeuse has a very young bow shock,
hence the star may have only recently become a RSG. To test this possibility we
calculated a stellar evolution model for a single star with properties
consistent with Betelgeuse. We incorporated the resulting evolving stellar wind
into 2D hydrodynamic simulations to model a runaway blue supergiant (BSG)
undergoing the transition to a RSG near the end of its life. The collapsing BSG
wind bubble induces a bow shock-shaped inner shell which at least superficially
resembles Betelgeuse's bow shock, and has a similar mass. Surrounding this is
the larger-scale retreating bow shock generated by the now defunct BSG wind's
interaction with the ISM. We investigate whether this outer shell could explain
the bar feature located (at least in projection) just in front of Betelgeuse's
bow shock.Comment: 5 pages, 3 figures; to appear in proceedings of the Betelgeuse 2012
Workshop, Paris, Nov. 201
ACS photometry of extended, luminous globular clusters in the outskirts of M31
A new population of extended, luminous globular clusters has recently been
discovered in the outskirts of M31. These objects have luminosities typical of
classical globular clusters, but much larger half-light radii. We report the
first results from deep ACS imaging of four such clusters, one of which is a
newly-discovered example lying at a projected distance of ~60 kpc from M31. Our
F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the
horizontal branch level, and clearly demonstrate, for the first time, that all
four clusters are composed of >10 Gyr old, metal-poor stellar populations. No
evidence for multiple populations is observed. From a comparison with Galactic
globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H]
< -1.8. The observed horizontal branch morphologies show a clear second
parameter effect between the clusters. Preliminary radial luminosity profiles
suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median
value of the globular cluster luminosity function. Our results confirm that
these four objects are bona fide old, metal-poor globular clusters, albeit with
combined structures and luminosities unlike those observed for any other
globular clusters in the Local Group or beyond.Comment: 12 pages, 3 figures, 1 table; accepted for publication in ApJ Letter
Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane
At least 5 per cent of the massive stars are moving supersonically through
the interstellar medium (ISM) and are expected to produce a stellar wind bow
shock. We explore how the mass loss and space velocity of massive runaway stars
affect the morphology of their bow shocks. We run two-dimensional axisymmetric
hydrodynamical simulations following the evolution of the circumstellar medium
of these stars in the Galactic plane from the main sequence to the red
supergiant phase. We find that thermal conduction is an important process
governing the shape, size and structure of the bow shocks around hot stars, and
that they have an optical luminosity mainly produced by forbidden lines, e.g.
[OIII]. The Ha emission of the bow shocks around hot stars originates from near
their contact discontinuity. The H emission of bow shocks around cool
stars originates from their forward shock, and is too faint to be observed for
the bow shocks that we simulate. The emission of optically-thin radiation
mainly comes from the shocked ISM material. All bow shock models are brighter
in the infrared, i.e. the infrared is the most appropriate waveband to search
for bow shocks. Our study suggests that the infrared emission comes from near
the contact discontinuity for bow shocks of hot stars and from the inner region
of shocked wind for bow shocks around cool stars. We predict that, in the
Galactic plane, the brightest, i.e. the most easily detectable bow shocks are
produced by high-mass stars moving with small space velocities.Comment: 22 pages, 24 figure
No sign (yet) of intergalactic globular clusters in the Local Group
We present Gemini Multi-Object Spectrograph (GMOS) imaging of 12 candidate intergalactic globular clusters (IGCs) in the Local Group, identified in a recent survey of the Sloan Digital Sky Survey (SDSS) footprint by di Tullio Zinn & Zinn. Our image quality is sufficiently high, at ∼0.4–0.7 arcsec, that we are able to unambiguously classify all 12 targets as distant galaxies. To reinforce this conclusion we use GMOS images of globular clusters in the M31 halo, taken under very similar conditions, to show that any genuine clusters in the putative IGC sample would be straightforward to distinguish. Based on the stated sensitivity of the di Tullio Zinn & Zinn search algorithm, we conclude that there cannot be a significant number of IGCs with MV ≤ −6 lying unseen in the SDSS area if their properties mirror those of globular clusters in the outskirts of M31 – even a population of 4 would have only a ≈1 per cent chance of non-detection
Young accreted globular clusters in the outer halo of M31
We report on Gemini/GMOS observations of two newly discovered globular
clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at
a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc,
onto a field halo substructure known as the South-West Cloud. We measure radial
velocities for the two clusters which confirm that they are almost certainly
physically associated with this feature. Colour-magnitude diagrams reveal
strikingly short, exclusively red horizontal branches in both PA-7 and PA-8;
both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the
morphology of the horizontal branch is maximally sensitive to age, and we use
the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both
objects are very likely to be at least 2 Gyr younger than the oldest Milky Way
globular clusters. Our observations provide strong evidence for young globular
clusters being accreted into the remote outer regions of M31 in a manner
entirely consistent with the established picture for the Milky Way, and add
credence to the idea that similar processes play a central role in determining
the composition of globular cluster systems in large spiral galaxies in
general.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Deterministic Brownian motion generated from differential delay equations
This paper addresses the question of how Brownian-like motion can arise from
the solution of a deterministic differential delay equation. To study this we
analytically study the bifurcation properties of an apparently simple
differential delay equation and then numerically investigate the probabilistic
properties of chaotic solutions of the same equation. Our results show that
solutions of the deterministic equation with randomly selected initial
conditions display a Gaussian-like density for long time, but the densities are
supported on an interval of finite measure. Using these chaotic solutions as
velocities, we are able to produce Brownian-like motions, which show
statistical properties akin to those of a classical Brownian motion over both
short and long time scales. Several conjectures are formulated for the
probabilistic properties of the solution of the differential delay equation.
Numerical studies suggest that these conjectures could be "universal" for
similar types of "chaotic" dynamics, but we have been unable to prove this.Comment: 15 pages, 13 figure
Quantum mechanics explained
The physical motivation for the mathematical formalism of quantum mechanics
is made clear and compelling by starting from an obvious fact - essentially,
the stability of matter - and inquiring into its preconditions: what does it
take to make this fact possible?Comment: 29 pages, 5 figures. v2: revised in response to referee comment
N-body Models of Extended Clusters
We use direct N-body simulations to investigate the evolution of star
clusters with large size-scales with the particular goal of understanding the
so-called extended clusters observed in various Local Group galaxies, including
M31 and NGC6822. The N-body models incorporate a stellar mass function, stellar
evolution and the tidal field of a host galaxy. We find that extended clusters
can arise naturally within a weak tidal field provided that the tidal radius is
filled at the start of the evolution. Differences in the initial tidal
filling-factor can produce marked differences in the subsequent evolution of
clusters and the size-scales that would be observed. These differences are more
marked than any produced by internal evolution processes linked to the
properties of cluster binary stars or the action of an intermediate-mass black
hole, based on models performed in this work and previous work to date. Models
evolved in a stronger tidal field show that extended clusters cannot form and
evolve within the inner regions of a galaxy such as M31. Instead our results
support the suggestion many extended clusters found in large galaxies were
accreted as members of dwarf galaxies that were subsequently disrupted. Our
results also enhance the recent suggestion that star clusters evolve to a
common sequence in terms of their size and mass.Comment: 12 pages, 8 figures, accepted by MNRA
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